Magmatic fabric acquisition mechanisms in a syenite: Results of a combined anisotropy of magnetic susceptibility and image analysis study

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Abstract
The structural significance of the anisotropy of magnetic susceptibility (AMS) and acquisition mechanisms of silicate and ferrimagnetic mineral fabrics are investigated in the Archean Lebel syenite intrusion, Ontario, Canada. Bulk mineral subfabrics are determined by intercept counting of mineral phases in classified digital images of thin sections cut parallel to the principal planes of the magnetic fabric ellipsoids of the samples. Intensities, shapes and principal direction orientations of the resulting intercept ellipsoids are generally similar to the corresponding AMS ellipsoids. Differences are due mainly to bimodal distribution of mineral fabrics observed on foliation planes of some samples showing pre‐full crystallization textures, and structural overprinting in samples deformed after crystallization. Subfabrics of clinopyroxene and magnetite in samples showing pre‐full crystallization microstructure are investigated by measuring inertia tensors of individual grain shapes. The resulting inertia tensor ellipsoids indicate that magnetic fabrics in these rocks are strongly correlated to the shape preferred orientation of magnetite, which is similar to the overall petrofabric of the samples. Crystal size distributions of clinopyroxene and magnetite indicate that they formed during in situ crystallization and that magnetite probably appeared late in the crystallization and flow history of the magma. Analysis of preferred orientations of these grains suggests that the petrofabrics of the samples were acquired during flow with strongly interacting grains, analogous to a logjam in a river. The final characteristics of the AMS and silicate petrofabric are the end points of a continuous crystallization and flow history in the magma, rather than transient late features.